The present invention pertains to printing machines in general. More particularly, the invention relates to a safety device for preventing entering of a foreign element into a contact gap between counter rollers of a printing machine.
Safety devices for printing machines of the type under consideration are known in the art. One of such devices is disclosed in the patent DD-PS No. 86022. The device described in the above patent is arranged in the vicinity of the contact gap between the printing rollers and includes a radially movable control shaft rotationally-rigidly connected to two guard plates and provided with a cam segment. The cam segment cooperates with a plunger of the control switch. The control shaft is held in its operation position by a spring-loaded rod.
Each guard plate is subdivided into two portions so that the outer portion of the plate faces the printing rollers whereas the inner portion thereof is pivotally supported and spring-loaded on a hinge.
If a foreign element enters the contact gap in the region of the printing rollers this foreign element presses against the guard plate. The safety arrangement upon that pressure turns and actuates the control switch through the cam element and shuts down the printing machine. Before the machine is finally stopped, the foreign element, for example a hand of an operator, must be pulled out. The outer portion of the guard plate will rotate against the force of the spring and will free a required space to enable one to pull the foreign element out of the gap.
The disadvantage of this otherwise satisfactory device is that when the guard plate is swung away from the printing roller the gap between the outer portion of the guard plate and the roller is still narrow. Although the required sufficiently large gap can be obtained by twisting or turning of the outer portion of the guard plate this will result in considerable high energy consumption during pulling out of the foreign element from the gap.
The outer portions of the guard plates in the known device should be locked in the open position when the printing machine is at standstill in order to enable an operator to make certain auxiliary operations, for example to replace the rubber blanket or to clamp the printing plate. A potential danger in this is that, after all the auxiliary operations have been completed the safety device is not closed, this causing an additional source of danger when the machine is put into operation. Furthermore, there is a possibility that during the above mentioned auxiliary operations a second person can switch on the main switch.
A further disadvantage of the known safety device is that its components, particularly the hinges of the guard plates, are subject to contamination.